Adding GradientDescent optimzer to ImageRegistrationMethod

Change-Id: If96b2acac6600ad4bfa076ea8f41dabeb28d6dbd
Adding MutualInformation metric to ImageRegistrationMethod

Change-Id: I3084069349cf1ac0d88716dc2411c9ddece70183
Adding ImageRegistrationMethod2 for Python

This is based on the ImageRegistration2 example in ITK.

Change-Id: I1e626ae5b6fb1a82250481ebe0a2f5b805b2e9dc
Add ImageRegitraionMethod2 for CXX

This example utilizes the MutualInformation metric with a
GradientDescent optimizer. It also feature required preprocessing.

Change-Id: I51bb600f948a4b13a5813f0f642ca6f7e5102b4e
Merge topic 'SIMPLEITK-521_AddInitialRegistrationComponents' into next

526d57a6 Add ImageRegitraionMethod2 for CXX
184fbb24 Adding ImageRegistrationMethod2 for Python
436a5b8b Adding MutualInformation metric to ImageRegistrationMethod
8961b54f Adding GradientDescent optimzer to ImageRegistrationMethod